Copper-Organic Framework Fabricated with CuS Nanoparticles: Synthesis, Electrical Conductivity, and Electrocatalytic Activities for Oxygen Reduction Reaction

To apply electrically nonconductive metal–organic frameworks (MOFs) in an electrocatalytic oxygen reduction reaction (ORR), we have developed a new method for fabricating various amounts of CuS nanoparticles (nano‐CuS) in/on a 3D Cu–MOF, [Cu3(BTC)2⋅(H2O)3] (BTC=1,3,5‐benzenetricarboxylate). As the amount of nano‐CuS increases in the composite, the electrical conductivity increases exponentially by up to circa 109‐fold, while porosity decreases, compared with that of the pristine Cu‐MOF. The composites, nano‐CuS(x wt %)@Cu‐BTC, exhibit significantly higher electrocatalytic ORR activities than Cu‐BTC or nano‐CuS in an alkaline solution. The onset potential, electron transfer number, and kinetic current density increase when the electrical conductivity of the material increases but decrease when the material has a poor porosity, which shows that the two factors should be finely tuned by the amount of nano‐CuS for ORR application. Of these materials, CuS(28 wt %)@Cu‐BTC exhibits the best activity, showing the onset potential of 0.91 V vs. RHE, quasi‐four‐electron transfer pathway, and a kinetic current density of 11.3 mA cm−2 at 0.55 V vs. RHE. A new method for fabricating CuS nanoparticles in/on a Cu‐MOF was developed. The conductivity of the composites is up to 109‐fold higher than that of the pristine MOF. They also show excellent activities for the oxygen reduction reaction, affording more positive onset potentials, higher electron transfer numbers, and greater kinetic current densities than the pristine MOF.